I visited the security.ubuntu.com/ubuntu/pool/main/etc... to find out that the libavdevice53_0.8.4-0ubuntu0.12.10.1_i386.deb wasn't present (hence the error) but other versions of that package were present. Same for all 3 missing packages.

I'm sure it is not a mint problem but maybe someone has a solution for me

The flightgear package doesn't depend explicitly on an obsoleted version, at least not on Linux Mint 14. Have you refreshed your package indexes? Software Manager and Update Manager do that for you; from the terminal you need to remember to do that yourself (no hand-holding here ). I'm not sure if Synaptic does this at startup, but it has a "Reload" button that will refresh the package indexes. Or from the terminal: